Without limiting the scope of the invention, its background is described in connection with existing radiant tube supports for industrial direct fired heaters or furnaces. Fired heaters are central to the processes required for the operation of oil refineries and the design of such heaters must conform to API 560. Fired heaters are also used in chemical and petrochemical processes to induce chemical reactions within the organic materials and are typically referred to as industrial fired furnaces. A fired heater typically consists of a radiant section, radiant coil, burners and a flue gas stack, and can also incorporate a convection section with coil, forced draft and/or induced draft fans or combustion air preheat. Each individual heater is designed to transfer a defined amount of heat to the coils at a certain rate, based on the volume and rate of organic material running through the coils all at a specified efficiency.
Depending on the chemical process, such heaters operate in the temperature range from 400° C. to over 1000° C. As depicted in FIGS. 1A-D, such heaters will generally include a radiant section 20 or firebox where heat is transferred from burner elements typically arrayed along a floor or side wall of the heater radiant section to organic materials that are conveyed in a continuous path through a multiplicity of radiant tube coils 22. The interior walls of the heater casing are lined (protected) with ceramic fiber insulation or castable refractory lining 30, which is designed to reduce heat loss, radiate heat back toward the back (shadowed) side of the tube coils and to keep the temperatures of the casing as designed. For efficiency purposes, the heaters will typically include a convection section 10 where heat is transferred to the conveyed organics in convection coils 12 through the process of convective heat transfer as heated flue gases rise out of the firebox, through the convection section and up and out a flue gas stack 15. Heater designs include those with horizontal radiant tubular coils and vertical radiant tubular coils. The radiant tubular coils can receive radiant heat from one side (single fired) or from both sides (double fired). The four most common heat designs utilizing single fired radiant tubes include vertical cylindrical helical coil fired heaters (FIG. 1A), vertical cylindrical vertical coil fired heaters (FIG. 1B), horizontal tube cabin (box) fired heaters (FIG. 1C), and horizontal tube twin cell (box) fired heaters (FIG. 1D).
Regardless of the configuration, the severe operating conditions required by the design include high operating temperatures that adversely affect the constituent metals of the entire internal aspects of the heater including by oxidation, creep, stress fracturing and thermal cycling during start up and shut down operations. The radiant tubes are further affected by the chemicals conveyed within them and deposits thereof including corrosion, erosion, metal dusting and coke deposition on the inner walls of the tubes. Coke deposits can result in elevated tube metal temperatures, leading to carburization and metal dusting of steel tubing, affect heat flow through each tube and the entire coil. Internal corrosion in the tube coils results from the combined effects of the chemical composition of the process fluid, process and tube metal temperatures, fluid velocity and tube metallurgy. Fortunately the radiant tubes can be inspected by several non-destructive techniques including by smart pigging, boroscoping, magnetic testing of austenitic tubes, ultrasonic testing, infrared inspection and by eddy current analysis, among others.
In fired heaters the radiant tubes are typically supported by radiant tube coil supports 24 (also termed hangers or hooks) that are generally bolted or clipped to the interior walls and roof of the heater. Due to the weight that they support, the radiant tube supports are generally bolted through the heater casing into structural support columns or beams 26 along the sides or roof, exterior to the heater casing. Insulation is installed throughout the interior of the heater radiant firebox, convection section, and typically the stack. The radiant tube supports hold the radiant coils in position by supporting and guiding the tubes and are designed to minimize sagging, bowing and buckling of the tubes and to keep the coil stable without swaying and within acceptable stress limits while allowing for the free expansion of the tubes of the coil during startup and shutdown of the heater. The coils are generally hung a distance off of the insulation interior surface (hot face) of the heater to reduce shadowing of the back side of the coil tubes, increasing heat transfer to the tube through improved radiation. The radiant tube supports are individually designed such that as a whole a plurality of supports will carry a vertical load that includes the weight of the tube and fitting components of the entire coil, the contents of the coil (fluid or gas), and the horizontal frictional load experienced when the coils expand/contract upon the heater being started/shutdown.
An example of a radiant tube support in use in the industry is shown in FIGS. 2A-C in top, side elevation and sectional views. The supports are typically of solid construction, consisting of an integral cast base 36 with holes 34 that are utilized to allow attachment to the firebox inner wall (by bolting through the wall). Typically, existing supports such as that depicted in FIG. 2A-C, include an arm portion 38 upon which the radiant tube 22 rests. The arm portion typically has a “T”, “I” or “[” cross section (consisting of a central web and a upper and or lower flange). The radiant tube supports are bolted from the inside of the radiant firebox to the inside of the casing wall 32 in uniform rows, spaced a distance apart, that distance being dependent upon size of tube being supported, and as dictated by applicable codes and specifications. Integral in the design in many of the radiant tube hook supports are keepers, a device that is intended to further guide the tube during operation, reducing the possibility of the tube lifting up and off of the support during heater operation. These keepers 42 typically consist of a separate cast pin of circular or rectangular cross-section that can be inserted vertically in a hole or slot to restrict the tube movement vertically and or horizontally. These keepers can attached to a single tube support hook or several depending upon the design. Tube supports such as those depicted in FIGS. 2A-C include an integral cast shoulder 44 that together with the keeper, control the lateral position of the tube coil on the support.
Radiant tube supports are made from heat resistant alloys chosen for high-temperature strength, creep properties, and resistance to corrosion. Typically, radiant tube supports are cast out of high nickel chrome alloy material capable of resisting the elevated temperatures experienced inside the radiant firebox enclosure. Nonetheless, the tube supports are susceptible to high temperature oxidation, high temperature embrittlement, thermal fatigue, stress fracturing and mechanical damage from thermal cycling during operations as well as corrosion from fuel ash and other combustion products. Tube hangers and supports fail for several reasons including elevated stress, long term creep, mechanical damage from vibration or expansion and mechanical impact during cleaning, corrosion, metallurgic changes over time, and poor quality castings.
As is clear from FIGS. 1A-D, a large number of tube supports are utilized to support the radiant tube coils. These supports are critical to the function of the heater because failure of one or more of the tube supports allows for sagging of the radiant coils or movement of the tube into the burner flames and may lead to catastrophic failure of the entire heater if the tube coils fail during operation. Current tube supports can only be inspected or changed out when the heater is non-operational. Changing out of a tube support is done from the interior of the heater because the tube supports are bolted through the heater casing from the interior of the heater. When replacement of tube supports are required, the supports must often be ordered as custom casting or fabrications with significant delay in supply because most industrial direct fired heaters are custom one off designs. As the different processes within an oil refinery or chemical plant cannot operate if the heater is out of service, any down time for the heaters will significantly affect the economics of the overall plant.
From the foregoing it is apparent there is a need in the art for improved tube supports that can be inspected and replaced during operation of the heater as well as tube supports that are universally adaptable for different heater designs. The invention described provides novel radiant tube supports that may be tested for integrity and replaced during heater operation and further provides a tube support system that may be retrofitted or replaced on existing heaters of varying designs while they remain in service.